Drip Irrigation Flow Calculator: GPM and Runtime
Planning a drip zone? Enter the emitters in a zone and their rating, your supply flow and how much of it you are willing to use. You get the zone flow in gpm or L/min, the maximum emitters per zone and the runtime to apply a target depth.
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Formula
zone flow = emitters x rating, then gph / 60 = gpm or L/h / 60 = L/min (1 US gal = 3.785411784 L)max emitters = floor(supply x share / flow of one emitter)volume (US) = area (sq ft) x depth (in) x 144 / 231gallons;volume (metric) = area (m2) x depth (mm)litresruntime = volume / zone flow
Worked example
100 emitters at 1 gph: 100 / 60 = 1.67 gpm (6.31 L/min). Supply 10 gpm at 80%: 8 / (1 / 60) = 480 emitters at most. A 500 sq ft zone needing 1 inch: 500 x 1 x 144 / 231 = 311.69 gal, and 311.69 / 1.667 = 187 minutes.
Metric case: 40 emitters at 4 L/h flow 2.67 L/min. A 20 m2 zone that needs 25 mm takes 500 L, so 500 / 2.67 = 187.5 minutes.
When to use it
Use it to split a garden into zones so that no zone asks for more water than the tap or pump can give. Run it again when you add emitters, change emitter ratings, or move from beds to a lawn. The share of supply you use is a safety margin; 75 to 80 percent is a typical choice.
Common mistakes
- Mixing up gph and gpm. Emitter ratings are usually per hour, supply is usually per minute.
- Entering supply flow measured at the tap when the pressure at the zone is lower.
- Using depth in the wrong unit: inches for US, millimetres for metric.
- Treating the runtime as a watering schedule. It is the time to apply a target depth at full flow, not a plan for how often to water.
How it differs from nearby calculators
The lawn watering calculator estimates how much water a lawn needs and how long sprinklers must run. The rain barrel calculator measures how much rain you can collect. This page is about the pipework side: flow, number of emitters per zone and runtime.
Frequently asked questions
How do I calculate drip zone flow?
Multiply the number of emitters by the rating of each, then convert to per-minute. 100 emitters at 1 gph is 100 gph, which is 100 ÷ 60 = 1.67 gpm (6.31 L/min).
How many emitters can one zone run?
Divide the supply flow you are willing to use by the flow of one emitter. With 10 gpm of supply at 80%, 8 gpm ÷ (1 gph ÷ 60) = 480 emitters. Using 75 to 80% of the measured supply is a typical margin.
How long should the zone run?
Runtime is the water volume needed divided by zone flow. For 500 sq ft at 1 inch you need 500 × 1 × 144 ÷ 231 = 311.7 gallons, which at 1.67 gpm takes about 187 minutes.
Is this a design guarantee?
No. It is a planning estimate that ignores pressure loss, uneven emitters and evaporation. Check pressure and the manufacturer data before building a system.
How many gpm is 50 drip emitters rated at 2 gph?
50 emitters at 2 gph is 100 gph in total. Divide by 60 to get 1.67 gpm, which is 6.31 L/min.
How many 4 L/h drip emitters can a 30 L/min supply run?
At 80 percent use, 30 x 0.8 = 24 L/min. One emitter flows 4 / 60 = 0.0667 L/min, so the supply can run at most 360 emitters.
How long to water 20 square metres with 40 emitters at 4 L/h?
The zone flows 40 x 4 / 60 = 2.67 L/min. To apply 25 mm over 20 m2 you need 20 x 25 = 500 L, and 500 / 2.67 = 187.5 minutes.